1. Field of the Invention
This invention relates to a nitride compound semiconductor light emitting element and its manufacturing method. More specifically, the invention relates to a nitride compound semiconductor light emitting element made by stacking GaN, InGaN, GaAlN and/or other nitride compound semiconductor layers on a substrate and having an electrode with a high adhesive force and a low contact resistance, a manufacturing method thereof, and a light emitting device including same.
2. Description of the Related Art
Nitride compound semiconductors expressed by Al.sub.x Ga.sub.y In.sub.1-x-y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, x+y.ltoreq.1) have been remarked for years as materials of light emitting diodes (LED) for ultraviolet light to blue or green light. Compound semiconductors of these materials have enabled intensive emission of ultraviolet light, blue light, green light, and so on, which was difficult before. These nitride compound semiconductors are usually grown on sapphire substrates which are insulating substrates. Therefore, unlike GaAs light emitting elements, electrodes cannot be made on bottom surfaces of substrates, and both anode and cathode electrode must be made on semiconductor layers stacked by crystalline growth.
Especially in case of semiconductor elements using nitride compound semiconductors in which sapphire substrates are translucent to emission wavelengths, they are usually mounted to orient the electrodes downward so as to take out light from the sides of the sapphire substrates.
FIG. 7 is a diagram schematically showing construction of an existing nitride compound semiconductor light emitting element. In the element, a GaN buffer layer 2, n-type GaN layer 3 and p-type GaN layer 4 are stacked on a sapphire substrate 1 by crystalline growth, and the p-type GaN layer 4 is partly removed by etching to expose the n-type GaN layer 3. The p-side electrode 15 is made on the p-type GaN layer 4, and the n-side electrode 14 is made on the n-type GaN layer 3. The light emitting element is mounted on a lead frame 17 by orienting the surface with the electrodes downward and using a conductive adhesive 16 such as silver paste.
In the existing semiconductor light emitting element shown in FIG. 7, light generated by injecting an electric current from the p-type GaN layer 4 to the n-type GaN layer 2 is reflected by the p-side electrode 15 and taken out through the sapphire substrate 1. However, the conductive adhesive 16 is apt to spread into a gap between the lead frames 17 (shown at A in FIG. 7) and a gap at the p-n junction (shown at B in FIG. 7), and liable to short-circuit the electrodes and the junction. It is therefore a problem of the existing semiconductor light emitting element that the production yield of the mounting process of the elements decreases seriously and the element is liable to deteriorate in the long-term which may cause a reliability problem.
The existing semiconductor light emitting element involves another problem that the need for a high accuracy in positional relation between the lead frames and the element inevitably decreases the productivity. This problem can be removed by increasing the element size to enlarge the distance between the lead frames 17, but it results in decreasing the number of chips obtained from a wafer and increasing the cost so much.
On the other hand, in order to decrease the forward voltage of a light emitting element, its electrodes must be in ohmic contact with the gallium nitride semiconductor layer. In existing light emitting elements, electrodes containing aluminum (Al) were used as n-side electrodes, and electrodes containing nickel (Ni) and gold (Au) were used as p-side electrodes. These electrodes, however, were not reliable in ohmic contact, and Al and Ni spread into GaN layers and degraded the reliability.